The present invention relates to automatic control devices for the adjustment of a physical parameter. We provide a process and an apparatus of mechanical coupling between the servomotor (actuating a driving rod having a linear motion) and a regulating valve (similarly actuated by a driven rod having a linear motion) of a device for the adjustment of the amplitude of a physical parameter.
The purpose of this coupling apparatus is to ensure a continuous adjustment, over a wide band, of the variation range of the parameter regulated by the said device and, more especially, of the nominal parameter of said range (i.e. of the maximum value of the regulated parameter), in accordance with the imposed working conditions.
Most mechanical regulating devices for the adjustment of a parameter comprise essentially a regulating system (or regulating valve) acting on the variation of the amplitude of the parameter and a servomotor for controlling the position of the said regulating valve, the said servomotor being actuated by means of an auxiliary power source such as pressurized air, pressurized oil, or electricity.
The regulation of the parameter is correlated with a control signal emitted by a regulator and transmitted to the servomotor.
The regulating apparatus is provided with a servo-system called positioner whose aim is to ensure the correlation between the control signal and the state of the regulating system. This is provided because usually the power of the control signal is insufficient for controlling the servomotor directly, and, for ensuring a great accuracy of the action of the servomotor in consideration of the level of the control signal. As any automatic servo-system, this servo-system conventionally comprises a reception device of the control signal emitted by the regulator; a perception device of the position of the regulating valve, in mechanical or analog form; a comparison device between a desired state of the regulation system, resulting (especially through a cam system) of the application to the control signal of a linear or non-linear operator and the actual state of the regulation system; and, finally, a power amplifier feeding the servomotor with auxiliary power, and whose action is controlled by the comparison device for reducing the discrepancy between the desired state and the actual state of the regulating system.
The most common and most advantageous application of this type of regulating apparatus, and more especially of the coupling device according to the present invention, is the fluid flow rate regulating valve which, therefore, will be taken as an example for the description of the invention although this should not be interpreted as limitating the scope of the invention.
In this application, the regulated parameter is the flow rate factor (hereinafter designated by FRF) of the valve connecting two parts of the piping through which flows a fluid.
Basically, the FRF of a valve is the number of U.S. gallons of water flowing through this valve in one minute when the pressure drop in the valve is one psi. It is therefore obvious that the FRF of a valve is a good representation of the restriction to the flow of a fluid which a valve opposes instantaneously.
In the course of this description a particular attention will be paid to valves whose regulation system is of the linear motion type, i.e. whose valve body connecting both ends of the piping comprises a transversal recess which houses a linearly guided assembly (seat and trap-valve) operated from the outside by the said driven rod.
The servomotor of these valves is usually of the pneumatic type (i.e. whose auxiliary power is produced by the pressure of a gas) and comprises a piston topped by the driving rod and lodged inside a cylindrical chamber against a membrane integrating the pressure action of the volume of gas introduced and confined in this chamber.
Finally, a coupling device made up of a more or less complicated system of levers ensures the transmission and amplification of the movement of the driving rod of the servo-motor to the driven rod of the variable obturation system.
This type of valve is at present widely used and appreciated in all industrial fields for effecting the automatic regulating of the rate of flow of fluids. It is not uncommon that one and the same plant uses several hundred valves effecting the regulation of the rate of flow of fluids of diverse types according to very different flow rate factors. However, this variety of service conditions (and of corresponding FRFs) raises many adaptation problems of the regulating valves for supplying, in each case, a valve with a nominal flow rate factor (i.e. a maximum FRF) exceeding as least as possible the computed working FRF.
This constraint in the adjustment of the nominal flow rate factor is imposed, on account of safety, in many fields such as the chemical one. As a matter of fact, it is obvious that a mechanical failure of a valve and especially of its servomotor may bring about a maximum opening of the obturation system of the valve; lead to chain incidents in the circuitry of the plant if the descrepancy between nominal flow rate and working flow rate is too great, and therefore result in serious accidents such as explosion or pollution.
Several methods are known at present for adapting the nominal flow rate factor of a valve to a predetermined FRF.
The simplest method consists in constructing a different valve for each case of application by homothetical reproduction of one of these valves. But this solution obviously leads to designing and manufacturing costs inconsistent with the present industrial constraints as each component of the valve must be manufactured individually.
A variant of this solution consists in reducing the stroke of the trap or poppet by altering the proper scale of the servomotor if the latter is not equipped with a positioner. This, however, leads to the same cost drawbacks as before.
Another variant, which is the most widely used solution at the present time, consists in providing, for the equipment of the valve body, a more or less comprehensive range of sets of variable obturation systems of the rate of flow (trap+seat).
The drawback of this solution is also its high cost price as such obturation systems are complicated components needing a very accurate machining and which would be more advantageously manufactured in mass production. Beside this, many applications need accurate differences between the working FRF and the nominal FRF of each valve. It is not uncommon, in particular, that the condition imposed to each valve of the installation is such as ##EQU1##
In such a case, the valve manufacturer, wishing (on grounds of cost and homogeneity of equipment to facilitate its maintenance) to equip the installation with one and the same type of valve, must keep in stock a range of obturation systems whose nominal FRFs are distributed according to a series of 1.2 geometrical ratio.
These conditions currently result in imposing a range of several hundred obturation systems for the equipment of one and the same plant.
Moreover, this latter solution, if adhered to by the manufacturer, also offers the drawback of being rigid and excluding the possibility of a fine and stepless adjustment of the range of nominal FRFs to particular conditions.
A second method, applicable mainly when the valves considered are equipped with positioners, consists in varying the feed-back scale of the positioner. As a matter of fact, the principle of working of a pneumatic positioner rests on the equilibrium of two forces the one exerted on a diaphragm by the pressure of the pneumatic signal from the output of the regulator and the other produced by the tension of a reaction spring whose length is related to the position of the servomotor rod, and therefore of the trap, through a reaction mechanism.
Consequently, the most common variants of this method consist either in using reaction springs of different stiffnesses, or in altering the kinematics of the movements between the reaction spring and the servomotor rod.
Contrary to the preceding methods, this solution offers the advantage of being simple and cheap and of permitting a stepless adaptation of the nominal FRF corresponding to the maximum signal. It has, however, two major drawbacks due to the facts that on the one hand, it is bound to the use of a positioner whose amplifying action of the signal is modified by the use of this solution, and, on the other hand, it is attended by great risks in the field of the safety of the installations as the nominal FRF of the corresponding valves is no more bound to a mechanical abutment (the extreme position of the servo-motor rod) but only to the extreme state of the control signal.
The second drawback practically excludes the application of this solution in all installations with a high safety-factor.
Finally, the applicant of the present Patent Application developed, in the past, a valve type whose nominal flow rate factor can be steplessly adjusted mechanically. The main purpose of this valve is to solve the difficult problem of the regulation of very low fluid flow rates under a very high pressure.
To achieve this purpose, that valve essentially comprises a frame having an axis of rotation; a primary lever hinged by one of its ends to the said rotation axis; an intergrated positioner-servomotor assembly, integral with the said frame and placed perpendicularly to the mean position of the primary lever, adjacent to its second end, and connected to the said second end by an axial driving rod in translation along a so-called first axis (of translation) perpendicular to the rotation axis; a secondary lever in the shape of a transmission rod hinged, (by one of its ends called upstream end), to a guiding saddle introduced inside a groove provided adjacent to the first end of the primary lever in a plane perpendicular to the axis of rotation, this saddle having besides a drilled-through longitudinal slot cooperating with a locking nut integral with the primary lever so that the position of the so-called upstream end of the secondary lever may be displaced and then locked in front of the corresponding end of the primary lever, and similarly hinged by its other end (called downstream end), in front of a secondary driven rod guided in translation along a so-called second axis of translation (intersecting the rotation axis, and perpendicular to the said rotation axis and to the first axis of translation) and actuating, by its movement, the trap of the valve located on its extension.
It is obvious that such a valve offers the following advantages, it may be switched from the "no-air opening type" to the "no-air closing type" by a mere displacement of the saddle inside its groove on both sides of the neutral position represented by the said second axis of translation; it allows a stepless adjustment of its nominal flow rate factor by altering the distance between the upstream end of the secondary lever and the neutral position; and finally it is very appropriate to the regulation of fluids under very high pressure as a consequence of the high reduction ratio between the driving (of the servo-motor) and driven (of the trap) translation movements which is offered by the orthogonal transmission coupling system formed by the primary and secondary levers coupled by two of their ends and mounted perpendicularly to the axis of the positioner, in extension to the axis of the trap.
However, the use of that type of valve is not widespread first of all because of the very high cost price of the coupling system due, in particular, to the difficulty of an accurate machining of the groove and saddle, and of the knee-joint-like hinges between the saddle, the secondary lever and the secondary rod, and to the very peculiar structure of this coupling system which forbids its adaptability to conventional valves.
Other disadvantages were the sharpness of adjustment of the FRF caused by the high reduction ratio and by the somwhat inadequate system of positioning of the saddle, and its very high space requirement as a consequence of the massive form of its two levers and of the arrangment of its auxiliary units (positioner and servomotor on the one hand, and flow rate regulating system on the other hand) according two orthogonal directions.
The present invention has for its general purpose to offer a relatively inexpensive means of using one and the same apparatus for the regulation of the amplitude of a parameter in a very wide field of application by permitting the adjustment of its nominal parameter to each of the imposed conditions of service.
Another object of the invention is to offer such a means of adjusting the nominal parameter of a regulating apparatus which solves the above-mentioned problems proceeding from the structure of the existing devices and which is simple, non-bulky, and easily adaptable to the majority of the existing regulating devices.
Still another object of the invention is to disclose a variant of such a means of adjustment of the nominal parameter of a regulating apparatus whose implementation is simple, rapid, continous and accurate.
A further object of the invention is to recommend several configurations of the means proposed which ensure, within a wide range of application, a quasi-linearity between the signal transmitted to the regulating apparatus and the action of this apparatus on the amplitude of the regulated parameter.
Still further the present invention is to describe, according to the principles set forth hereafter, a new idea of a fluid flow rate regulating valve with a stepless adjustment of its nominal flow rate factor, which is simple, non-bulky, accurate, and very reliable as regards the safety level.
With these aims and objects in view, the invention refers mainly to a process of mechanical coupling with adjustable reduction ratio between a rotary (or "driving") motion around a first fixed axis and a translation (or "receiving") motion.
This process is remarkable by the fact that on the one hand, the said receiving translation motion is activated, from the driving rotary motion, by coupling it mechanically with a second rotary (or "receiving") motion around a second fixed axis, and, on the other hand, an adjustable kinematic correlation is established between the driving and receiving rotary motions.
As a matter of fact, this process is appropriate to a very advantageous application for effecting a mechanical coupling according to a reduction ratio adaptable between two translation motions respectively called driving and receiving motions. This variant consists in controlling the driving rotary motion by coupling it mechanically with the driving translation motion.
It is easily understood that such a process enables to carry out, in an apparatus for the regulation of the amplitude of a parameter, a mechanical coupling between the servomotor imparting a translation motion to a driving rod, guided along a first axis, and the operative means of the said apparatus on the amplitude of the parameter, the said means being actuated by a receiving rod guided along a second axis.
It is also understood that, according to the selection of the mechanical correlation between the driving and receiving rotary motions, this process enables carrying out the adaptation of the kinematic reduction ratio of the coupling (i.e. the regulation range of the apparatus) over a wide band.
Besides this, the invention also refers to a device suitable for the application of a process such as the above-mentioned one for effecting a mechanical coupling with adjustable reduction ratio between a first driving rotary motion around a first fixed axis and a receiving translation motion.
Such a device comprises a first driving lever rotating around a first fixed axis for embodying the first driving rotary motion, a second receiving lever rotating around a second axis parallel to the first axis, and a receiving rod guided by and hinged to the said second lever for embodying the receiving translation motion. According to the invention, the second rotation axis is fixed, and the first and second levers are coupled together by a coupling means (e.g. an auxiliary part) integral with at least one adjustable fixed point of one of the said levers and defining a motion, assigned with a certain amount of freedom, between the said fixed point and the other lever.
Therefore the mere selection of the said fixed point defines the reduction ratio between the driving rotary motion and the receiving translation motion.
A very valuable application of this device is its use for the mechanical coupling between the translation-guided, driving rod of the servomotor of a fluid flow rate regulating valve and the driven rod of its trap or poppet.
As a matter of fact, in a widely used valve type, the driving rod of the servomotor and the receiving rod of the trap are parallel with one another and are both connected with a sole, lever-forming, control arm ensuring the amplification of the transmitted force.
This arm is mounted in nearly perpendicular relationship with the common direction of both rods and is pivoted around a fixed point near the hinged zone of the receiving rod.
It is, therefore, advisable to equip the above-mentioned type of valve with the coupling device according to the present invention. According to a preferred embodiment, both levers are mounted in a direction which is nearly perpendicular to that of the motion of both rods. This construction enables obtaining a regulating valve with adjustable nominal flow rate factor whose value is defined by an abutment of the piston of the servomotor inside its chamber and, therefore, with a very high safety factor.
Besides this, it offers, over the existing valves with adjustable nominal factor, the advantages, of being of much simpler construction, of requiring less space because of the collinearity between the servomotor, the valve body and possibly (as hereafter explained) the positioner, and of being easily adaptable, without major alteration, to the existing types of valves.
According to an advantageous variant of the coupling process of the invention, a fixed point (called connection point) is selected on one of the two levers and then, in the course of the movement of the two levers, this point is compelled to follow a path which is interdependent with the other lever.
More specifically, according to a variant recommended by the invention, the driving and receiving levers are both provided with an elongated passage. An axle for the transmission of the movement between the two levers extends through the elongated passage of each lever. This axle is provided with a blocking system of the locking and unlocking type which fixes it in a determined position in relation with one of the two levers. The passage of the other lever, besides, has a width equal to the cross-section of the said transmission axle so that the free portion of this transmission axle can slide without clearance inside this passage.
It is therefore possible to operate a stepless adjustment of the reduction ratio (and, more particularly, of the nominal flow rate factor of a valve) by a mere displacement of the position of the axle along the lever to which it is bound.
It is besides recommended that the passage of that lever, along which the axle of transmission of movement slides freely, be rectilinear. Moreover, both the driving and receiving levers will preferably be straight and nearly parallel to one another during their movement. This construction ensures a quasi-linear demultiplication between the driving and receiving motions. This feature is much appreciated in the case of a regulating valve as it ensures a proportionality between the control signal and the position of the trap.
Similarly, the invention locates each of the axes of rotation of both levers in the vicinity of the receiving rod in order to increase the reduction ratio between the driving motion and the receiving motion and, therefore, the sensitivity of the apparatus.
Besides, according to a very useful variant of application, one at least of the two levers is provided with an elongated graduated scale in front of which an adjusting mark, interdependent with the axle of transmission of the movement between the two levers, is moved so as to enable a stepless and scaled adjustment of the demultiplication of the coupling. Preferably, the elongated scales of the flow rate regulating valves will be graduated in percentages of the maximum nominal flow rate of the valve.
Finally, in case the regulating apparatus, and especially the regulating valve, is provided with a positioner bound to the frame of the apparatus for establishing a bond of dependency between the control signal and the state of the regulation system by seizing the movement of this system through a feed-back spring, the invention advises to bind the end of this spring to the driving lever of the coupling device.
The scaling of the positioner is thus made independent from the adjustment of the nominal flow rate factor and the linearity between the action of the positioner and that of the control signal is mainained.
Other characteristics and advantages of the invention will appear from the following description and appended related drawings, the said description and drawings being submitted as non-limiting examples.